FR2546910A1 - PROCESS FOR THE PREPARATION OF SQUARIC ACID BY ELECTROLYTICALLY TETRAMERIZING CARBON MONOXIDE IN ANHYDROUS ALIPHATIC NITRILE AS A SOLVENT - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR PREPARING SQUARIC ACID BY ELECTROLYTIC TETRAMERIZATION OF CARBON MONOXIDE IN ANHYDROUS ALIPHATIC NITRILE AS A SOLVENT. SQUARIC ACID, ITS COMPLEXES AND OR SALTS ARE PREPARED BY PERFORMING A REDUCING ELECTROLYSIS OF CARBON MONOXIDE UNDER PRESSURE, UNDER CONDITIONS OF SEPARATION OF ANODIC AND CATHODIC REACTIONS, USING AS A SOLVENT AT LEAST ONE ANODIC AND CATHODIC REACTIONS, AS A SOLVENT AT LEAST ONE ANYHYL 2 AN ALIPDRA A A NITRIANT 8ATOMS OF CARBON. THE PRODUCT IS OBTAINED IN A SOLID FORM EASY TO SEPARATE. THE RESULTING COMPOUNDS CAN BE USED AS INTERMEDIARIES IN THE PREPARATION OF COLORANTS AND OR POLYMERS, AND AS SEQUESTRANTS.

Description

The present invention relates to a process for the preparation of "the acid

  squaric "(dihydroxycyclobutenedione), compound having the formula:

O = C C OH

  -o = C C OH as well as the preparation of its complexes and salts More

  In particular, the present invention relates to the preparation

  of these compounds by electro-cyclotetramerisation

  reducing carbon monoxide in solvents

  consisting of one or more anhydrous aliphatic nitriles.

  The resulting compounds have potential utility as intermediates in the preparation of dyes, polymers, products for destroying viruses,

and as sequestering agents.

  The first report on the synthesis of squaric acid (I) was presented in 1959 by S Cohen, J R. Latcher and JD Park in JA Chem Soc, volume 81, page 3480. These authors have described the hydrolysis of certain derivatives halogenated cyclobutene Squaric acid has a particularly interesting chemistry, partly due to its dianion (II): ## STR1 ## which can be considered as a tetrameric dianion

  carbon monoxide, and which has an electro-

  completely delocalized So, although by nature

  "phenolic", the acid is strong (p K 1 = 0.6, p K 2 = 3.4).

  US-A-3,833,489 (1974) describes a process for preparing squaric acid, its complexes and its salts and shows in a brief summary the state of the art at the time. The process involves passing a electric current in a solution of carbon monoxide in a solvent medium selected from the group consisting of

  amides of phosphoric acid, amides of aliphatic acids,

  carboxylic acids having 1 to 10 carbon atoms, aliphatic ether oxides, cyclic ether oxides, liquid polyethers, and anhydrous ammonia, at a

  temperature between about 30 ° C and a temperature of

  ture up to the boiling point of said solvent and at pressures up to about 420 bar, thereby to cause electrolytic reductive cyclotetrameration of carbon monoxide at the cathode, the reaction being carried out under separation conditions. The aforementioned patent has attempted to claim, as a functional group of solvents, all the non-aqueous solvents which conduct the current while presenting a minimum resistance, but the actual work. inventors of said patent has revealed that only certain amides, ethers and ammonia can be used,

  and that many other classes of compounds are ineffi-

  In addition, their system is seriously hampered by the fact that a subsequent separation of the squaric product from the reaction system is quite difficult and, thus, an industrial use of the described prior system presents difficulties. Other articles of the same researchers (Gazetta Chimica Italiana, vol 102,

  p 818-821 (1972) and Electrochimica Acta, vol 23, p 413-

  417 (1978) also investigated the influence of specific parameters, such as the particular solvent, electrolyte, electrode material, carbon monoxide pressure, and reaction temperature on squaric acid yield. have determined that there is a great degree of unpredictability in their process, particularly with regard to the properties of the particular solvent used. It is particularly interesting to note their discovery that solvents such as acetonitrile gave poor results ( about 2% current yield), which led to the conclusion that nitriles are not effective as solvents for the production of squaric acid-based compounds. An additional annoying problem, especially in a large-scale industrial operation. is that the separation of the products of the squaric acid type, or derived therefrom, from the resulting residue is It is furthermore found that when using the preferred class of solvents claimed in the aforementioned US Pat. No. 3,833,489, it is also found that when using solvents such as dimethylformamide (DMF) it is found that squaric acid, it may result, surprisingly, large fluctuations in product yields, even in the case of experiments substantially

identical from one end to the other.

  An object of the present invention is therefore to develop a simple, efficient and economical process for the preparation of squaric acid, its metal complexes and its salts, by electrochemical reductive cyclotetramerisation of carbon monoxide in anhydrous solvents of the aliphatic nitrile type. , consistently obtaining high yields of product through operations

  relatively simple isolation and extraction of the product.

  Another object of the present invention is to provide a method of preparation and recovery

  squaric compounds, making recovery much easier.

  subsequent products and allowing the reuse of

  starting materials not consumed.

  It can therefore be stated, in summary, that the invention involves an improved process for the preparation and recovery of squaric acid, its complexes and salts, by the passage of an electric current, for example and preferably a continuous current. although an alternating current may be used in a carbon monoxide solution maintained in a temperature range corresponding to the range in which the particular solvent is liquid, and in a compression range of from about 1 to 420 bar and preferably between about 30 and 150 bar, for performing the cathodic reductive cyclotetramerisation, by electrolysis, of carbon monoxide, the process comprising carrying out the reaction in anhydrous solvent (s) selected from a class of aliphatic nitriles containing each of 2 to about 8 carbon atoms, the most preferred solvent being isobutyronitrile The passage of the electric current This reaction causes the reduction of carbon monoxide to the C 4042 squarate ion, the reaction being carried out under almost total separation of the anodic reactions, or their reaction products, or the non-interference of the anodic reactions or the reaction products. anodic with the reactions

  cathodic or the products of the cathodic reaction.

  After completion of the reaction, isolate by centrifugation

  filtration or solids containing almost all the squarate formed It is thus much easier to achieve

  and effectively that in previous systems the recovery of

  squaric acid, electrolyte and other

raw materials unaltered.

  Looking more closely, it can be seen that the electrochemical cyclotetramerization of carbon monoxide to the squarate ion has been examined and studied with considerable interest, since the reaction is based on a widely available raw material and inexpensive to achieve a final product that is a

  C 4 monomer molecule potentially useful for obtaining

  certain polyamide polymers.

  However, there are still no industrial processes useful for the production of a large volume of squaric compounds, which is evidenced by the current price being in the neighborhood of 17,600 F per 1 kg In the aforementioned US patent No. 3,833,489, as well as in the process of the present invention, the following reaction scheme apparently serves to form squaric acid, most often in the form of a metal squarate, or complex, insoluble or non-soluble reagent, using as a cation source M a soluble metal anode

O O O

1) 4 CO + 2 e-

o HQ no + Mn +

2) M O

O O

  22 + salt or squarate complex 3) | + 2 Mn N insoluble or non-reactive ano G> As in the case of the process of the aforementioned patent,

  the present process involves first creating an environ-

  significantly avoiding any oxidation in the anodic zone of carbon monoxide reaction products, as well as a reduction in the cathodic zone of these products.

  products obtained from the anodic reaction section.

  Thus, it is necessary to establish certain operating parameters in order to prevent the products of the anodic reaction and / or the anodic reaction itself from influencing substantially

  and embarrassingly the products of the catholic reaction

  or on the cathodic reaction itself, and vice versa.

  Such an absence of adverse influence (or such non-interference) can be obtained by choosing from a number of different conventional methods, some of which are described in the aforementioned US-A-3,833,489, the methods cited herein being By way of example, baffles, diaphragms or forced circulation of the solution inside the cell can be used, making a careful choice of conditions in order to obtain only the formation of chemically produced products.

  inert oxidation, or the formation of oxy-

  which are liquid and / or gaseous and / or are substantially

  non-electrolytes, and that the anolyte is then continuously removed.

  Of course, one can also combine two or more of these techniques.

  Although to a certain extent we can carry out the electrochemical reduction of CO in the squarate anion under conditions

  very diverse, for example using anodes of resistance

  different corrosion ("corrodible" or soluble, or non-corrodible, ie non-soluble) anode, using direct or alternating current, using different temperature and pressure conditions, and by varying the composition of the chemical solvent, the

  The present invention relates mainly to the perfection-

  surprisingly obtained because of the use of a particular class of solvents in the described system.

  in the aforementioned US Pat. No. 3,833,489 and in the

  It has been found that, contrary to the teachings of these reference documents, aliphatic nitriles containing from 2 to about 8 carbon atoms can be used as solvents in the aforesaid reaction and thus obtain particularly good results.

  In particular, it has been discovered that squamous acid is

  is generated in insoluble form, probably in the form of a metal salt, when carbon monoxide is electrochemically

  anhydrous consisting of one or more nitriles as sol-

  using metal anodes which are "soluble anodes"; the preferred nitrile as solvent is selected from the group consisting of isobutyronitrile,

  normal butyronitrile, propionitrile and acetonitrile.

  The best results are obtained when essentially anhydrous isobutyronitrile is used, and current yields of about 50% have been achieved.

  that other aliphatic nitriles may be

  economic considerations make it likely that

  the unlikely use, and aromatic nitriles do not seem to be as effective.

  in the formation of squaric acid in

  However, the nitrile solvents are particularly useful because the product formed squarate, which is obtained substantially in the solid state by the method of The present invention is much easier to separate by centrifugation, filtration or other separation techniques than the squarates formed, for example, in an amide medium. In addition, the best separation properties of the resulting mixture containing the product (s). allow the recycling of starting raw materials or raw materials unaltered, such as electrolyte, which allows to consider the implementation of the process in continuous form as well as discontinuous On the contrary, the product formed in the system described by the Italian has proved to be much more difficult to separate. So far, we have not obtained a simple separation the head of a dimethylformamide squarate, except by the distillation of the solvent, and this way of operating

non-volatile

  The process described here can be used with anode

  "corrodable" that is to say consumable or soluble, or with a non-corrodable anode, according to the teachings of the aforementioned patent US-A-3 833 489, but, in the preferred form of

  embodiment, it is desired to operate using a solubilized anode

  because of the ease and simplicity of the corresponding engineering and the implementation of this process. It has been found that, depending on the choice of the solvent used, the particular metal which

  Anode may be critical or fundamental.

  For example, squaric acid has been formed with 40 to 50% current efficiencies when magnesium or aluminum anodes are used, while barely a few hours have been achieved. acid

  squaric using titanium and that we have no abso-

  foolishly not obtained using mild steel Although we do not wish to bind or be bound by a theory,

  this is thought to be due to differences in solubility

  salt metal squarates formed in each soil.

  Indeed, an insoluble salt prevents the anodic oxidation of the squarate formed at the cathode Alternatively, these results can come from the oxidation potential differences of these anodic metals in the nitrile chosen as a solvent, since the metal must oxidize more easily than any soluble squaric salt, to avoid anodic oxidation of squarate Although the precise mechanism is not certain, it is thought that the conditions existing at the anode are probably due to some combination of

  less one or two of these factors.

  particularly well suited to serve as soluble or consumable metal anodes in aliphatic nitriles as solvents are aluminum, magnesium and tin anodes, as well as alloys and / or blends of

  these metals, and in particular aluminum and magnesium

  Sium, while titanium and iron have proven not to be effective Other metals may be effective and are within the scope of the present invention 2 and may include copper, lead, zinc, lead indium

and other analogous metals.

  On the contrary, it has been found that

  the cathode only slightly influences the electrolytic reaction

  It is possible to form suitable cathodes from steel and aluminum alloys and / or their mixtures, steel being particularly useful. However, in the widest embodiment, it is possible to use

  almost any material like cathode.

  In order to increase the conductivity of the solution, one can add to it one or more auxiliary electrolytes, such as a tetraalkylammonium halide or other electrolytes described as being useful in the

  aforementioned patent US-A-3 833 489 Tetra-

  alkylammonium are the most effective.

  The current density used in the electrolysis reaction can vary over a wide range depending on the particular parameters that are used. The electric current that is applied can be current.

  continuous or alternating, and the direct current is preferred.

  The temperature of the reaction system may be in the entire range corresponding to the liquid state of the particular solvent being used, and may be, for example, between a suitable temperature.

  above the freezing point and the corresponding temperature

  boiling at the boiling point of nitrile or nitriles

  As solvents, it is particularly preferred to

  first a temperature range of about 10 to 50 ° C

  can operate the system under the pressures of

  between atmospheric pressure and about 420 bar, and pressures of between about 30 and 150 bar are particularly preferred although, within certain limits, the conversion rate obtained is all the greater the better the pressure is. A particularly interesting aspect of the invention is the surprising unpredictability of the efficiency or performance of a particular solvent. It has been discovered that many of the solvents claimed in the above-mentioned US-A-3,833,489 are substantially ineffective, of

  same as several polar solvents common in electro-

  chemistry, such as propylene carbonate and sulfolane.

  The following examples are presented for

  to illustrate but not to limit the invention.

  In the following examples, the precise amount of squaric acid present is determined by applying high pressure and ultraviolet liquid chromatography techniques using an "Aminex HPX 87" column (Bio Rad Laboratories) with

  mobile phase formed by H 2 SO 4 0.001 N (flow rate: 0.6 ml / min).

  The temperature of the column is maintained at 650 ° C. The squaric acid is detected by spectrophotometry at 270 nm. The retention time is approximately 6 to 7 minutes. The apparatus described in Example 1 also serves for Examples 2,

3, 4 and 9 to 14.

EXAMPLE 1

  The present example illustrates the oligomerization of CO to squaric acid in isobutyronitrile as a solvent with Bu 4 N Br as electrolyte and an aluminum anode under a CO pressure of about 70 bar. Isobutyronitrile and 3 ml are introduced. 0 g of Bu 4 N Br in a Parr bomb equipped with a magnetic stirring bar An aluminum rod is connected via an electrical bulkhead connection to the positive pole of a source of electrical energy The bomb is tightly sealed, it is connected to the negative pole of the energy source, and introduced into CO to a pressure of about bars is applied a DC current (approximately m A) up to a load corresponding to 18.6 m F.

  The gas is released into the atmosphere and separated by centrifugal

  fugation of the electrolysis mixture the resulting solids washed with isobutyronitrile and dried (2.81 g) The solids analysis shows that they contain 12.82% by weight

  squaric acid (0.36 g, current yield: 34%).

EXAMPLE 2

  The present example illustrates the sequence of CO, or its oligomerization, in squaric acid in isobutyronitrile with Bu 4 NI as electrolyte; The mixture is stirred under a CO pressure of approximately 70 bar of isobutyronitrile and 4.0 g of Bu 4 NI and a direct current (approximately 100 m A) is applied until a change in

  load corresponding to 24.8 m F The gas is released to the atmosphere

  The resulting solids were filtered off from the electrolysis mixture and washed with isobutyronitrile and dried (2.69 g). The analysis of these solids shows that they contain 22.04% by weight. weight of squaric acid (0.59 g;

current efficiency: 42%).

EXAMPLE 3

  The present example illustrates the sequence of CO, or its oligomerization, in squaric acid in a solution, made especially anhydrous, Bu 4 NI in isobutyronitrile.

  5.0 g of Bu 4 NI are dissolved in 100 ml of isobutyl alcohol.

  ronitrile and this solution is kept on 4 A activated sieves for 4 days in a dark room 60 ml of the dehydrated electrolyte solution are then stirred under a pressure of about 70 bars of CO, and there is A direct current (about 100 m A) is passed through to about 24.0 m F of charge. The gas is vented to the atmosphere and the resulting solids are filtered off and dried. air (2.59 g) Analysis of these solids shows that they contain 23.8% by weight of squaric acid (0.62 g;

current efficiency: 45%).

EXAMPLE 4

  This example illustrates the sequence of CO, or its oligomerization, in squaric acid in

  wet isobutyronitrile comprising Bu 4 N Br.

  60 ml of isobutyronitrile, 0.5 ml of distilled H.sub.2 O and 3.0 g of Bu.sub.4 N.sub.Br are stirred under about 70 bar of CO, and direct current (about m.sup.-1) is passed through. 15.9 m F. The gas is then released into the atmosphere and the electrolysis mixture is analyzed for squaric acid content (0.005% by weight, 0.002 g). performance

current: 0.2%).

EXAMPLE 5

  This example illustrates the sequence or oligomerization of CO to squaric acid using

a magnesium anode.

  The same apparatus as in Example 1 is used, except that an anode is used as a magnesium rod rather than an aluminum rod. At a CO pressure of approximately 70 bar, 60 ml of isobutyronitrile is stirred. and 3.0 -0 g Bu 4 N Br, and passing a direct current of about m A until passing a load corresponding to 26.0 m F.

  The gas is released into the atmosphere and separated by centrifugation.

  electrolysis mixture the resulting solids

  washed with isobutyronitrile and dried (3.53 g).

  The analysis of these solids shows that they contain 11.48% by weight of squaric acid (0.41 g, current yield).

27%).

EXAMPLE 6

  The present example illustrates the sequence or oligomerization of CO to squaric acid, using Bu 4 NI as an electrolyte with a magnesium anode under a

CO pressure of about 97.5 bar.

  The same apparatus as in Example 1 is used, but replacing the aluminum rod with a magnesium rod as the anode. 60 ml of isobutyronitrile (distilled and dehydrated) is stirred under a CO pressure of 97.5 bar. activated sieves 4 A) and 2.0 g of Bu 4 NI, and the applied

2546910 '

  a continuous current of about 100 m A until passage

  a load of approximately 27.2 m F The gas is released to the atmosphere.

  The resulting solids were filtered off from the electrolysis mixture and dried under a stream of air (2.36 g). Analysis of these solids showed that they contained 27.54 wt. squaric acid (0.65 g, current yield: 42%) The filtered electrolyte solution does not contain squarate and is then used without

  additional treatment, in Example 7.

EXAMPLE 7

  This example illustrates the sequence or oligomerization of CO to squaric acid in a solution

Electrolyte already used.

  The same apparatus as in Example 6 is used. The filtered electrolysis solution used in Example 6 is stirred under a CO pressure of approximately 97.5 bar and a direct current of about 100 is applied. The gas is released to the atmosphere and the resulting solids are filtered off from the electrolysis mixture and dried under a stream of air. 2.46 g are obtained. Analysis of these solids shows that they contain 25.82% by weight.

  squaric acid (0.63 g, current yield: 50%).

  The filtered electrolyte solution does not contain squarate.

EXAMPLE 8

  This example illustrates the sequence or oligomerization of CO to squaric acid when using

a titanium anode.

  The same apparatus as in Example 1 is used, replacing the aluminum rod with a titanium rod as the anode. 60 ml of isobutyronitrile and 3.0 g of Bu are stirred under a CO pressure of approximately 60 ml. 4 N Br, and a continuous current of about 100 m A is applied for 6 hours. The gas is released into the atmosphere and the mixture is then analyzed.

  electrolysis to determine the level of

  There is a content of 0.016% by weight, ie

0.0081 g.

EXAMPLE 9

  This example illustrates the sequence of

  CO or its oligomerization to squaric acid in propionic acid

nitrile as solvent.

  A pressure of 70 bar CO of about 60 ml of propionitrile and 3.0 g of Bu 4 N Br is stirred and a direct current of 100 m 3 is applied initially until a charge corresponding to 35.0 m F The gas is released into the atmosphere and the resulting solids are separated by centrifugation from the electrolysis mixture.

  washed with propionitrile and dried, 3.86 g are obtained.

  Analysis of these solids shows that they contain 8.36% by weight of squaric acid (0.32 g, current yield:

16%).

EXAMPLE 10

  This example illustrates the sequence or

  oligomerization of CO to squaric acid in acetoin

  nitrile. 60 ml of acetonitrile and 3.0 g of Bu 4 N Br are stirred under a CO pressure of approximately 70 bars, and a continuous stream of approximately 200 m A is applied for 5 hours. the atmosphere and then analyze the mixture

  electrolysis to determine the level of

(0.31% by weight, -0.16 g).

EXAMPLE 11

  This example illustrates the sequence of

  I oligomerize CO to squaric acid in butyro-

normal nitrile.

  60 ml of normal butyronitrile and 3.0 g of Bu 4 N Br are stirred under a CO pressure of approximately 70 bar and a direct current (approximately 100 m A) is applied until a charge is passed through. corresponding to 11.7 m F The gas is released into the atmosphere and the resulting electrolysis mixture is analyzed for squaric acid content.

  (0.20% by weight, 0.10 g, current yield: 16%).

EXAMPLE 12

  This example illustrates the sequence or

  oligomerization of CO to squaric acid in pivalonic acid

  Trile. The mixture is stirred under a CO pressure of about 70 bar of pivalonitrile and 3.0 g of Bu 4 N Br, and 5.5 h of DC is applied for about 5.5 m.

  gas to the atmosphere and analyze the mixture of electro-

  resulting lysis to determine the acid content

  squaric (0.08% by weight, 0.04 g).

EXAMPLE 13

  This example illustrates the sequence of oligo-

  merger of CO with squaric acid in valeronitrile.

  The mixture is stirred under a CO pressure of approximately 70 bar ml of valeronitrile and 3.0 g of Bu 4 N Br, and a continuous stream (30 to 100 m A) is applied until a

  charge corresponding to 10.3 m F The gas is released to the atmosphere

  The resulting solids were filtered off from the electrolysis mixture and washed with valeronitrile and dried (0.93 g). Analysis of these solids showed that they contained 5.82% by weight of squaric acid (0.05 g;

current efficiency: 9.2%).

EXAMPLE 14

  This example illustrates the sequence of oligomerization of CO to squaric acid in benzonitrile.

  Trile. The mixture is stirred under a CO pressure of approximately 70 bar of benzonitrile and 3.0 g of Bu 4 N Br, and 5.5 h of DC current is applied for approximately 100 m.

  gas to the atmosphere and analyze the mixture of electro-

  resulting lysis to determine the level of

  There is no evidence of squaric acid.

  It goes without saying that, without departing from the scope of the invention,

  Many modifications can be made to the process described for preparing squaric acid or

its derivatives.

2 4691 O

Claims (13)

  A process for preparing squaric acid, its complexes and, optionally or alternatively, its salts, comprising passing an electric current through a solution of carbon monoxide which is maintained at a temperature between the point of freezing point and the boiling point of the particular solvent present, the solution being further maintained at a pressure between atmospheric pressure and approximately 420 bar, this being   which causes electrolytic reductive cyclotetrameration   carbon monoxide at the cathode, the reaction being carried out under conditions of almost complete separation or non-interference of anodic reactions   and anodic reaction products with respect to   cathodic reactions and cathodic reaction products, characterized in that the solvent used is at least one anhydrous aliphatic nitrile containing about 2 to 8 carbon atoms.   2. A process according to claim 1, characterized in that the nitrile solvent is selected from the group consisting of isobutyronitrile, butyronitrile   normal, propionitrile and acetonitrile.   Process according to Claim 2, characterized   in that the solvent is isobutyronitrile.   4 process according to claim 1, characterized   in that DC is used as the electrical current   cudgel. Process according to Claim 1, characterized in that the anode is made of a conductive metal   consumable or soluble in the environment of electrolysis.   Process according to claim 5, characterized   in that the conductive metal is selected from the group consisting of aluminum, magnesium, tin, and   * alloys and mixtures of these metals.   7 Process according to claim 5, characterized in that the cathode is made of a substantially non-corrodible or soluble conductive metal and which is essentially inert   from the chemical point of view in the conditions of electro-   lysis. Process according to Claim 1, characterized in that the separation of the catholyte from the anolyte is carried out by means of baffles or diaphragms. 9 Process according to claim 1, characterized in that the separation of the catholyte from   the anolyte by circulating each fluid separately.   Process according to Claim 1, characterized in that the reaction products in the catholyte are non-interfered with those obtained in the anolyte by forming products which are chemically   inert under the conditions of electrolysis.   Process according to Claim 1, characterized in that the reaction products formed in the catholyte are non-interference with those formed.   in 11 anolyte by the formation of products which are isolate in the reaction medium.   Process according to Claim 1, characterized in that the reaction is carried out at a temperature of between approximately 100 ° C. and approximately 500 ° C. The process according to Claim 1, characterized in that the reaction is carried out at a pressure of between about 30 and 150 bar.   Process according to claim 1, characterized   in that the anodic oxidation products are liquid.   Method according to claim 1, characterized   in that the anodic oxidation products are gaseous.   Process according to claim 1, characterized in that the anodic oxidation products are substantially non-electrolytes.   Process according to Claim 1, characterized in that the reaction mixture is separated by filtration,   squarate or squaric acid products formed.   Method according to claim 1, characterized   in that the reaction mixture is separated by centrifugation   tion, squarate or squaric acid products formed.   19 Process according to claim 1, characterized in that after the preparation of squaric acid can reuse the electrolyte.